In the study of the seismic behaviour of structures, the Soil Structure Interaction (SSI) is concerned with the interaction between the soil, from which come the solicitation, and the structure (building, industrial installation, ...). The SSI can be considered as a kind of multi physic problem. The physical phenomena for the soil or the structure are described by the same equations. But, for the soil, the best point of view is to consider wave’s propagation in an infinite medium, whereas for the structure, the most suitable approach is, in many case, to use eigen modes on the modal basis. The currently used numerical methods for SSI consider two different domains, with interactions at the interface. Boundary Element Methods (BEM) and Finite Element Methods (FEM) can be used. An application is presented, with the interpretation of test performed in Japan, for a Structure Soil Structure Interaction (SSSI) problem, considering the interactions between two adjacent buildings during an earthquake. The tests were performed by JNES in Japan, and the interpretations presented here are performed by CEA, in France, using FEM methods. The field test experiments have been carried out by NUPEC (JNES) under different conditions with one building, two identical buildings or two different buildings in an excavation, for the “surface configuration”, and in the “embedded configuration”, when the excavation is filled. Forced vibration test and earthquake observations are being carried out in the field test. NUPEC proposed a theoretical model for the interpretation of the experimental results, including soil and buildings mechanical characteristics. The results obtained with this model are similar to the experimental ones. Sensibility analyses have been developed, based on the NUPEC theoretical model, for the forced vibration tests and the seismic motion. It is possible to reproduce, with numerical simulations, the fact that, for two buildings, the movement under a seismic solicitation is a little lower for two adjacent buildings than for one isolated building.